As tetraethyl lead is phased out, oxygenates have become more important in the petroleum refining industry as a source of gasoline octane boosters. The most common oxygenates for this purpose are the dialkyl ethers, especially those in the C5 to C7 range. One such dialkyl ether that is generating much interest is diisopropyl ether (DIPE). DIPE is in the boiling range of gasoline, has a high blending octane number, and one reactant generally used in the formation of DIPE, propylene, is a by-product commonly available in refineries. The preparation of DIPE from propylene proceeds by two sequential reactions, where propylene is first hydrated to isopropyl alcohol (IPA) (1) followed by reaction of the alcohol with the olefin (2) or by a single bimolecular dehydration reaction of the alcohol (3) (Williamson synthesis) according to the equations,

These reactions are catalyzed by a variety of catalysts such as activated charcoal, clays, resins, and zeolites. In particular, the reactions may be catalyzed by acidic ion exchange resins including sulfonated cation exchange resins such as sulfonated polystyrene resins and sulfonated styrene/divinylbenzene co-polymers as disclosed in U.S. Ser. No. 08/079,768, G.B. 1,176,620, and U.S. Pat. No. 4,182,914. Halogenated strong acid ion exchange resins such as those described in U.S. Pat. Nos. 4,705,808, 4,269,943, and 3,256,250 also may be used. A recognized problem of these catalysts is their susceptibility to hydrolysis of the acidic groups causing the transfer of acidic material from the catalysts into the reaction mixture and ultimately into the reactor effluent. The hydrolysis depends strongly on the reaction temperature, and the higher the temperature the greater the degree of hydrolysis. Steps may be taken to remove acid from process streams to protect downstream process units.
The propylene-containing hydrocarbon feedstock may be a refinery C3 hydrocarbon stream and will most likely be a mixture of propylene and propane. Previously, the propylene-containing hydrocarbon feedstock typically contain at least about 50 mass-% propylene, or from about 70 to about 80 mass-% propylene. Sources for the propylene-containing hydrocarbon feedstock have included gas plant off-gas containing propylene, naphtha cracker off-gas containing light olefins, propylene from a propane dehydrogenation process, and refinery fluidized catalytic cracked (FCC) propane/propylene streams.
Recently, high purity propylene feedstocks have become available where the feedstock comprises from about 90 mass-% to about 99.9 mass-% propylene. Therefore there is a need for a process for generating dialkyl ethers using high purity propylene feedstocks.